The present invention is directed to a method for the transmission of a high-bit-rate digital signal.
Subscriber line modulesxe2x80x94also called line cardsxe2x80x94of the new generation offer the possibility of transmitting digital signals at high speed with the assistance of innovative transmission methods. This is achieved in that the assemblies can transmit or receive in a significantly greater frequency range than was previously employed for the analog audio signals. Given an ADSL (asymmetrical digital subscriber line), the digital signals are transmitted, for example, in a frequency range of approximately 30 kHz through 1.1 MHz. Transmission rates of up to 8.192 Mbit per second can thereby be achieved, so that such transmission methods could also be employed by call service (for example, video on demand). ADSL, but also UDSL (universal digital subscriber line) as well, thereby offer the possibility of also transmitting the signals of a standard telephone connection (plain old telephone service, POTS) or, respectively, of an ISDN terminal in a base frequency band on the same line parallel to the high-bit-rate digital signal.
In the simultaneous transmission of the signals of the standard telephone connection and of the high-bit-rate digital signal, however, special situations can occur wherein the high-bit-rate data traffic is deteriorated by signals of the voice band. Such a special situation can occur, for example, in the transmission of ringing signals or of meter pulses. For the transmission of a symmetrical ringing signal, audio frequencies at the level of 20 or 25 Hz that are offset in phase by 180xc2x0 are transmitted on each of the two leads of the two-lead connecting cable, namely with a voltage of more than 100 V. In the case of an asymmetrical ringing signal, a sine signal is transmitted with 20 or 25 Hz on one of the two leads, whereas the other lead is grounded. In both instances, however, the level of the transmitted ringing signal is higher than 100 V. In the transmission of meter pulses, sine frequencies with 12 or 16 kHz are transmitted, for example, on both leads, whereby these sine signal on the two leads are phase-offset by 180xc2x0 and the voltages amount to about 5 Volts.
Due to the transmission of such special signalsxe2x80x94particularly of the ringing signals with their higher voltagesxe2x80x94, there is the risk that the phase synchronization between the subscriber line module and a modem of the subscriber side that receives the digital signal will be disturbed or even lost. This then leads either to a noticeable increase in the bit error rate for the duration of this special situation or, on the other hand, also leads to a complete abort of the connection. In this case, the data traffic then only resumes after a renewed, complete connection setup, which lasts approximately 10 seconds.
It is therefore an object of the present invention to specify a method for the transmission of a high-bit-rate digital signal between the subscriber line module of a public switching center and a modem of the subscriber side, whereby the signals of an analog standard telephone connection can also be transmitted over the same telecommunication line parallel to the digital signal, and whereby the quality of the data connection is enhanced.
According to the method of the invention for transmission of a high-bit-rate digital signal between a subscriber line module of a public exchange and a modem of a subscriber side via a telecommunications line, signals of an analog standard telephone connection are transmitted parallel to the high-bit-rate digital signal. Before occurrence of a special situation that influences the transmission of the high-bit-rate digital signal, with the subscriber line module or the subscriber-side modem outputting an information signal. The transmission of the high-bit-rate signal is modified for a duration of the special situation due to the information signal. This inventive method is characterized in that, before the occurrence of a special situation, i.e., for example, before the transmission of a ringing signal or of a meter pulse, the subscriber line module or the subscriber-side modemxe2x80x94dependent on the line end from which the signal influencing the transmission of the high-bit-rate digital signal is suppliedxe2x80x94outputs an information signal, and the transmission of the digital signal is modified in a suitable way for the duration of the special situation on the basis of this information signal. This means that both devices that are responsible for the sending and receiving of the digital signal are set to a problematical data transmission during this time and, for example, activate correction possibilities or specific filters. The transmission rate of the digital signal can then in fact still drop slightly, but a complete connection abort and a subsequent, complete connection setup procedure can be avoided in any case or at least is clearly shortened, so that the quality of the data connection clearly increases.
The information signal is output by that device from whose end the infeed of the signal that is problematical for the high-bit-rate transmission occurs, since this device can already be informed in advance about the occurrence of the special situation. As a rule, thus the infeed of the information signal occurs proceeding from the exchange side. However, it would be conceivable that the modem of the subscriber side is likewise connected to the telephone of the subscriber side and is informed when the setup of the telecommunication connection occurs proceeding from the subscriber side, so that the information signal can also be output proceeding from the subscriber side.
The information signal can assume the greatest variety of forms. When, for example, the transmission of the high-bit-rate signal ensues according to the ADSL method, then a number of possibilities are available. The International Telecommunication Union ITU is currently deliberating the passage of a new standard (G.992.1) for the transmission of signals according to the ADSL method. A similar standard has already been passed by the American National Standards Institute ANSI (T1.413). The European standard of the ITU defines a number of possibilities for exchanging messages between the modem of the exchange sidexe2x80x94i.e. the line card or the assemblyxe2x80x94and the modem of the subscriber side. These possibilities offer several degrees of freedom for defining new messages. In the ADSL method, for example, the transmission of the data thus occurs within what are referred to as super frames wherein a plurality of what are referred to as indicator bits (IB) are respectively provided, these containing information about the status of the devices participating in the data traffic. An employment has thereby not yet been provided for some of these indicator bits, so that these can be employed, for example, for the implementation of the inventive method. Currently, the first seven indicator bits are not yet occupied, so that an indication could be provided, for instance by setting the indicator bit 0, that a ringing signal is about to be forwarded onto the line or that a ringing signal is being transmitted at the moment.
Furthermore, the transmission of EOC (embedded operation channel) messages with which specific functions can be activated is provided in the framework of the data exchange according to the ADSL method. Here, too, functions have not yet been assigned to a number of EOC messages (for example, the codes 15, 16 or 80), so that these are also available for the transmission of the inventive information signal. The third possibility is comprised in defining new AOC (ADSL overhead control channel) messages that likewise indicate the occurrence of a special situation.
The three possibilities that have just been described refer specifically to the data transmission according to the ADSL method. Both in this method as well as in the UDSL method, the frequency range in which the transmission of the digital signal ensues is divided into a plurality of sub-channels. This is referred to as DMT method (discrete multi-tone). The subdivision of the frequency range is thereby regulated by specific DMT parameters. These parameters also offer the possibility of employing them for the transmission of the information signal. For example, the phase or the amplitude of a pilot tone that is responsible for the time-synchronization of the two data transmission and reception devices could be modified for this purpose. In the same way, a further synchronization symbol (sync symbol) that characterizes the recognition of the start and the end of a data packet could also be modified. Furthermore, certain control signals (cyclic prefix) precede every data block. These, too, could be modified in a suitable way for indicating a special situation.
A further possibility is comprised in informing the opposite terminal device of the information about the occurrence of a special situation within the framework of the payload data transmitted in the transmission method. However, the information signal could also be formed by a new audio signal whose frequency lies outside the frequency range employed for the transmission of the digital signal. This audio signal could then be correspondingly modulated in order to indicate the special situation.